Use CA certificates from standard location in Brillo.

Brillo images have all the CA certificates for the whole system in
/system/etc/security/cacerts. For update_engine, we would like to
accept only a sub-set of those, but we will start by accepting all
the installed certs.

Bug: 25089263
Test: `mm`, deployed on a -userdebug build and updated

Change-Id: Ib05e6ea1c5efb98a6320ce6288842e2277f67b1f
diff --git a/platform_constants_android.cc b/platform_constants_android.cc
index b6ee29f..02889eb 100644
--- a/platform_constants_android.cc
+++ b/platform_constants_android.cc
@@ -27,6 +27,7 @@
 const char kOmahaPlatformName[] = "Brillo";
 const char kUpdatePayloadPublicKeyPath[] =
     "/etc/update_engine/update-payload-key.pub.pem";
+const char kCACertificatesPath[] = "/system/etc/security/cacerts";
 // No deadline file API support on Android.
 const char kOmahaResponseDeadlineFile[] = "";